The present invention relates to an electron imaging device, and more particularly to an improved gateable intensifier tube, or electronic image devices such as electron bombarded camera tubes.
An image intensifier is an electron device which produces an observable image and is brighter at output than the optical image at the input. The image intensifier converts an optical input image to an electron image, and back to an optical image while intensifying the original image. Image intensifiers are typically used as night vision devices, or in other low light level applications, such as low light level television camera tubes. The intensification of the image is achieved by typically accelerating the electrons, which are a representation of the optical image, by applying a very high voltage to an accelerating anode. The electrons imaged upon the output means have increased energy in the form of kinetic energy. These high energy electrons typically impact a phosphor screen at the output means to produce photons which exceed the number of electrons which were initially released from the photocathode surface at the input means. This brings about a luminous gain or a light amplification. An image intensifier tube is a hermetically sealed evacuated device, typically having fiber optic input faceplate with a photoelectric layer deposited on the interior surface of the input faceplate. The optical input information generates the release of electrons from this photoelectric layer, and these electrons are accelerated and focused by the device.
In some situations it is desirable to be able to gate the operation of such image intensifier tubes. A gated tube is a tube having a control electrode with the appropriate potential for electrically controlling the operation of the tube, typically in an on-off fashion, and to thereby electronically shutter the equivalent optical information. The control electrode is typically synchronized with the switching on of the tube with some external auxiliary circuitry. The external circuitry may be utilized to drive an illuminating light source, which may be pulsed to provide reflected light for viewing the scene under consideration. It may be also desirable to gate the operation of the electron image intensifier to permit selective viewing of the scene, for example if a high light intensity source should pass into the field of view, it may be desirable to gate the image intensifier to prevent the undesirable image persistence and obliteration of the scene which may be produced by such a high intensity source. One common present technique for gating such electron image intensifier tubes is to utilize a fine conductive mesh spaced proximate to the photocathode surface. The mesh is operated at a relatively low potential difference from the photocathode but at high absolute negative potential. Such mesh electrodes are a distorting influence upon the imaging process, because of the physical disposition of the mesh between the input image means and the output image means. The mesh is also typically fixed in place and produces a further distortion problem when the device is operated in the non-gated mode, that is when the mesh is maintained at the same potential as the photocathode surface. Such mesh gated image intensifier tubes are also objectionable because of the relatively high gate voltage required for operation, usually about 10% of the anode voltage. The output image is deteriorated as a result of the excessive noise during the gating operation when such large gating voltages are used. The typical anode voltages in such tubes range from 12 to 15 kilovolts, and thus, the gate voltage required is about 1200 to 1500 volts. With gate voltages in this range it is also very difficult to effect rapid switching from the non-gated to the gated mode of operation, and for accurate rapid synchronization of the gated operation with auxiliary systems.
Other electronic imaging tubes have similar input and output imaging means, with the output target being an array of diodes. This type of device is termed an electron bombarded camera tube. It is also desirable to be able to gate such devices while avoiding the inherent problem of using the standard mesh gate.